Medical institutions have a radiographic apparatus installed therein for taking images of a subject. As shown in FIG. 11, such a radiographic apparatus 51 includes a radiation source 53 for emitting radiation, and a radiation detector 54 for detecting the radiation.
The radiation source 53 includes a collimator 55 that can open and close freely to change spread of a radiation beam. The collimator 55 has a pair of leaves 55a. They approach and move away, thereby restricting a direction of radiation emitted from the radiation source 53.
When an operator instructs opening and closing of the collimator 55 via a console, the radiographic apparatus 51 calculates a movement extent of the leaves 55a. For instance, when a slider set on the console is operated, the radiographic apparatus 51 recognizes how many centimeters the leaves 55a are to be moved in accordance with a distance by which a lever of the slider moves, and moves the leaves 55a by the calculated distance relative to a drive controller for the leaves 55a of the collimator 55. See, Japanese Utility Model (Registration) Publication No. S62-195800.
The leaves are schematically displayed on a monitor attached to the radiographic apparatus 51. The leaves displayed on the monitor as a graphic symbol are moved in accordance with a degree of opening of the actual leaves 55a. Recognition of the leaves on the monitor by an operator can cause confirmation a of a restricted extent of a radiation beam. A position of the leaves 55a on this monitor is determined in accordance with the calculated degree of opening indicating how many centimeters the leaves 55a are moved.
When the operator gives instructions on radiation irradiation via the console, the radiation source 53 emits radiation. Here, the collimator 55a restricts the radiation beam. The restricted extent is same as that confirmed by the operator prior to radiation irradiation. As above, the leaves on the monitor are moved synchronously with the actual leaves 55a, whereby spread of the radiation beam upon radiation irradiation can be determined in advance.
The conventional construction, however, has the following problem.
The conventional radiographic apparatus 51 has a problem that the degree of opening of the collimator indicated by the leaves 55a displayed on the monitor differs from the degree of opening of the actual collimator 55. Such problem arises. Under this state, although the operator confirms the leaves 55a on the monitor, the operator cannot recognize accurately the restricted extent of an actual irradiation direction of the radiation beam.
Description will be given of how such inconsistencies in degree of opening of the collimator occur. For changing the degree of opening of the collimator 55, it is firstly determined how many centimeters the leaves 55a are to be moved corresponding to a distance by which a lever of a slider on a console moves. Subsequently, a drive controller for the leaves 55a of the collimator 55 moves the leaves 55a by the distance as determined. However, the leaves 55a are not always moved correctly by the distance as set.
There are two or more reasons for an inaccurate movement distance of the leaves 55a. Firstly, a mechanism for moving the leaves 55a causes an inaccurate movement distance of the leaves 55a. The leaves 55a are moved as a rotatable arm rotates. Consequently, the leaves 55a are not moved horizontally but are moved along an arc path close to a straight line. When the leaves 55a are moved by a given distance, the arm rotates by a given angle, accordingly. Thus, when the arm is moved by the same angle, a length of the path by which the leaves 55a follow through the movement is always the same. However, since the path is not a straight line, the collimator 55 does not always have the degree of opening as set. As noted above, the leaves 55a are moved along the arc path. Thus, although the arm is moved by the same angle, difference in initial position of the leaves 55a causes slight difference in movement distance of the leaves 55a. 
Moreover, it is difficult to position and attach the collimator 55 onto the radiation source 53 accurately. A slight deviation in position of the collimator 55 and the radiation source 53 from an ideal position will affect actual radiography sufficiently. Specifically, a slight deviation of the collimator 55 is enlarged as the radiation beam emitted from the radiation source 53 spreads toward a subject M. Consequently, although a deviation in fixing position of the collimator 55 and the radiation source 53 is around 0.1 mm, a position of the radiation beam emitted to the subject M may be order of magnitude of centimeters. In addition, a deviation in position of the radiation source 53 and the radiation detector 54 from an ideal position causes a deviation in relative position of the leaves 55a of the collimator 55 relative to both the components 53, 54. Accordingly, an ideal position of the leaves 55a on the monitor deviates from the actual position of the leaves 55a. 
Since the leaves 55a of the collimator 55 displayed on the monitor are moved in accordance with output from the drive controller for the leaves 55a, a position of the leaves 55a specified by the drive controller is displayed on the monitor. The actual leaves 55a of the collimator 55 are not always moved as the ideal. Consequently, inconsistencies in degree of opening occur between the actual collimator 55 and the collimator 55 on the monitor. Therefore, it is difficult for an operator to recognize an accurate irradiation area of the radiation beam prior to radiation irradiation.